Vesicular stomatitis virus (VSV) is a widely studied prototype virus that is currently being developed as a vaccine vector and as an oncolytic agent for the treatment of cancer in humans. VSV is a principal example of a virus that suppresses host antiviral responses through the global inhibition of host gene expression. Studies with VSV, which is a highly cytopathic virus, have also contributed fundamental insights into the mechanisms of virus-induced cell death. Both the suppression of host antiviral responses and the induction of cell death depend on the ability of the VSV matrix (M) protein to inhibit host gene expression. The goal of this project is to determine the mechanisms by which M protein suppresses host antiviral responses and the mechanisms by which M protein controls induction of cell death by VSV. VSV inhibits host gene expression at multiple steps, including transcription, translation and nuclear-cytoplasmic RNA transport. Specific Aim 1 is to determine how M protein suppresses host antiviral responses. In Aim 1a, levels of expression of Rae1, one of the cellular targets of M protein, will be altered in cells by siRNA and by overexpression of Rae1, and the effects on M protein-mediated inhibition of host transcription, RNA transport, and translation will be determined. In Aim 1b we will determine whether formation of the M protein-Rae1 complex involves post-translational modification of Rae1 and/or formation of a complex with other host cellular factors. These experiments are based on our new data indicating that only a minor population of Rae1 in cells is competent to form a complex with M protein. In Aim 1c, the affinity of a panel of M protein mutants for binding to Rae1 will be determined and correlated with the ability of the mutant M proteins to inhibit host transcription, RNA transport, and translation. Specific Aim 2 is to determine how M protein regulates the induction of apoptosis by VSV. Aim 2a will determine the role of upstream signaling molecules in the induction of apoptosis by VSV containing wt or mutant M protein. In Aim 2b the relative importance for viral oncolysis in vivo of the apoptotic pathways activated by VSV will be determined by treating tumors established from cells that express inhibitors of apoptosis with wild-type and M protein mutant VSV. These experiments will contribute fundamental new information on the cellular pathways by which viruses suppress antiviral responses through the global inhibition of host gene expression, and how the induction and suppression of antiviral responses leads to death of virus-infected cells. These experiments will establish general principles for viruses that globally inhibit host gene expression, and should also contribute to understanding the basis of tumor destruction by oncolytic viruses.